For many years, game balls such as baseballs and softballs have been formed by providing a spherical core of cork, kapok or other such material, placing a winding of string or yarn over the core to provide strength and resilience, and sewing a cover of leather or the like over the windings.
Many problems with these prior art balls have long been recognized. The balls vary widely in size and weight so that many balls are rejected. If the size is too far from standard a cover will not fit, and if the weight is too far from standard it will be unacceptable to the players. The prior art balls also tend to have a short playing life. The cover and windings may split from normal play, or the ball may become so badly non-spherical that playing characteristics are unacceptable. Additionally, the prior art balls will soak up water when exposed to wet, or even damp, conditions. When the ball becomes water-soaked, the weight of the ball is greatly increased and the resilience of the ball is decreased.
With the vast technology in plastic materials, there have been several balls formed of foamed plastics. These have taken the form of a molded sphere of expanded plastic material, the sphere being covered by the conventional leather cover, with or without an intermedate winding of string. These balls also have not met with great success. A ball made of ethylene vinyl acetate (EVA) has been attempted, but the coefficient of restitution of the ball is too great, which is to say the ball rebounds better than is desired. When hit by a baseball bat, the EVA ball travels so fast as to be a distinct danger to players. Also, when the EVA ball is cold, the ball becomes so dense as to bend a metal bat. A similar ball has been made of a cross-linked polyurethane, and this ball has been found to be affected by changes in climate so that the ball is not consistent. The polyurethane ball tends to take a compression set so that the sphericity is not guaranteed during play.
As a result of the above mentioned and other difficulties with the balls made of plastic, many of the balls in regular use are still made with the center of cork or the like covered by conventional string windings and the conventional leather cover.